366 CLASSICS OF MODERN SCIENCE 



the atmosphere the absorption takes place which produces the various 



phenomena on which the chemical classification has been based. 



It is obvious that the general spectrum of the sun, like that of stars 

 generally, is built up of all the absorptions which can make themselves 

 felt in every layer of its atmosphere from bottom to top ; that is, from 

 the photosphere to the outermost part of the corona. Let me re- 

 mind you that this spectrum is changeless from year to year. 



Now, sun-spots are disturbances produced in the photosphere ; and 

 the chromosphere, with its disturbances, called prominences, lies di- 

 rectly above it. Here, then, we are dealing with the lowest part of 

 the sun's atmosphere. We find first of all that, in opposition to the 

 changeless general spectrum, great changes occur with the sun-spot 

 period, both in the spots and chromosphere. 



The spot spectrum is indicated, as was found in 1866, by the widen- 

 ing of certain lines ; the chromospheric spectrum, as was found in 

 1868, by the appearance at the sun's limb of certain bright lines. 

 In both cases the lines affected, seen at any one time, are relatively 

 few in number. 



In the spot spectrum, at a sun-spot minimum, we find iron lines 

 chiefly affected ; at a maximum they are chiefly of unknown or unfa- 

 miliar origin. At the present moment the affected lines are those 

 recorded in the spectra of vanadium and scandium, with others never 

 seen in a laboratory. That we are here far away from terrestrial 

 chemical conditions is evidenced by the fact that there is not a gram 

 of scandium available for laboratory use in the world at the present 

 time. 



Then we have the spectrum of the prominences and the chromo- 

 sphere. That spectrum we are enabled to observe every day when 

 the sun shines, as conveniently as we can observe that of sun spots. 

 The chromosphere is full of marvels. At first, when our knowledge 

 of spectra was very much more restricted than now, almost all the 

 lines observed were unknown. In 1868 I saw a line in the yellow, 

 which I found behaved very much like hydrogen, though I could 

 prove that it was not due to hydrogen ; for laboratory use the sub- 

 stance which gave rise to it I called helium. Next year I saw a line 

 in the green at 1474 of Kirchhoff's scale. That was an unknown line, 

 but in some subsequent researches I traced it to iron. From that 

 day to this we have observed a large number of lines. They have 



